This invention relates to a method and apparatus for manufacturing a glass rod preform from which an optical light transmitting fiber can be drawn.
An optical communication fiber comprises a central core glass section and a surrounding cladding glass section having a lower refractive index than that of the core glass section. Such light transmitting glass fibers are broadly classified as either a step index type in which the radial refractive index distribution varies in a stepwise manner across the fiber, or a graded index type in which the distribution is parabolic.
A glass rod which can be drawn into a thin glass fiber is called a "preform", which also comprises the core glass and cladding glass sections. One method of manufacturing such preforms is disclosed in U.S. Pat. No. 3,823,995, in which a start member having a circular cross section is first prepared, and then glass "soot" particles made from raw glass gaseous materials by flame hydrolysis are laminated on the surface of the start member as it is being rotated. In this operation various dopants in addition to quartz glass are added to the laminated glass to vary its refractive index. The amounts of dopants are controlled for every layer of glass so that the resulting preform has a predetermined refractive index distribution. Thus, if the glass particles are first laminated with a constant dopant concentration, and the latter is then changed to abruptly decrease the refractive index, the resulting preform is of the step index type; if the dopant concentration is gradually changed so that the refractive index is decreased for each laminated layer, then a graded index type of preform is obtained. The soot-like glass lamina thus obtained is then placed in a high temperature furnace which renders it transparent, and the start member is thereafter removed. Only the cylindrical glass part is heated, and the hollow inner part is collapsed to form the preform.
Another conventional method is disclosed in U.S. Pat. No. 3,737,293, in which gaseous raw glass materials are successively coated on the inner wall of a quartz tube while it is being rotated and heated. In this method the dopant concentration is also controlled for each layer so that the final preform has a predetermined refractive index distribution. It is thereafter heated at a high temperature and the hollow part is collapsed to obtain the preform.
The above-described conventional methods are disadvantageous, however, in the following respects. In the method of U.S. Pat. No. 3,823,995, since the thermal expansion coefficient of the start member is different from that of the laminated glass, the laminated glass is liable to crack as a result of slight temperature variations or impacts during the forming process. In the method of U.S. Pat. No. 3,737,293, the length, diameter and wall thickness of the quartz start tube is limited, and therefore it is impossible to obtain a large preform.
U.S. Pat. No. 4,135,901 to Fujiwara et al discloses a preform manufacturing method which is completely different from the above-described methods. In the Fijiwara et al method a plurality of nozzles or burners jet gaseous raw glass materials toward a rotating start substrate or toward one end face of a rotating start member. The nozzles or burners are disposed parallel with the start member rotation axis or form an angle therewith so that the raw glass materials are oxidized or hydrolyzed in a high temperature atmosphere to synthesize glass particles. The glass particles are deposited on the start substrate or on the tip end of the rod-shaped start member, and are laminated in the direction of the rotation axis to thereby obtain the preform. In this method the dopant compositions in the gaseous raw glass material jetted by the plurality of nozzles or burners are controlled such that the refractive index of the central glass laminates close to the rotational axis of the rod are higher than those of the laminates more remote from the rotational axis. This method is not affected by the start member and is suitable for manufacturing large preforms.
Two important characteristics are required with respect to the transmitting surface of a light communication fiber. One is that the optical transmission loss be low. The other is that the transmitted light pulse waveform deformation and pulse width variations be minimal during transmission. The latter characteristic means that a wide transmission band of a base band frequency with a low loss is required. As the refractive index distribution of the fiber approaches a parabola the transmission band width of the base band frequency is increased. In general, there are three factors which increase the optical transmission loss. The first is the scattering of light which is caused by refractive index variations in the axial direction. The second is light absorption due to the presence of impurities and the third is the scattering of light which is caused by structural imperfections due to core diameter variations in the longitudinal direction.
With respect to the two above-described transmission characteristics, the method of U.S. Pat. No. 4,135,901 has certain disadvantages. For example, with a plurality of nozzles or burners, even if the start substrate or rod-shaped start member is rotated during the glass lamination process, the laminated dopant distribution is not completely uniform in the circumferential direction. Consider the case where the axis of a burner for synthesizing the core glass is positioned in coincidence with the rotational axis of the start member. In general, the volume or amount of glass laminated is 20-50 g/hour. If the lamination is carried out at a rate of 36 g/hour, or 0.01 g/sec., and this is converted into a fiber 150 .mu.m in diameter, then the amount of glass laminated corresponds to a fiber produced at a rate of 25 cm/sec. If the substrate or start member is rotated during the laminating process at one revolution per second, then any variation in the refractive index of the fiber will have a period of 25 cm/sec. since the dopant jetted by the burner for the cladding glass is spirally distributed with a period of 25 cm/sec. The same result is obtained even if another cladding glass burner is symmetrically disposed with respect to the rotational axis, and the problem cannot be solved by merely arranging a plurality of nozzles. The resulting non-uniformity of the refractive index in the axial direction increases the transmission loss.
The fact that the refractive index distribution varies spirally and periodically means that such distribution on any given section of the fiber does not have circular symmetry. That is, none of the refractive index distributions are identical or constant on straight lines parallel to the central axis of the preform, which reduces the transmission band width of a graded index type of fiber. Another disadvantage accompanying the use of a plurality of nozzles or burners is that the number of gases to be controlled is increased, and therefore the apparatus is necessarily bulky and intricate and it is difficult to manufacture preforms with high reproducibility or consistency.